Material composites made of graphite and high- melting metals, instead of high-melting metals only, have found increasing utility in a number of high-temperature applications. When compared to one-component materials consisting only of high-melting metals, these composite structures permit expanded high-temperature applications as a result of their superior heat storage capacity and also due to the lower specific weight of the graphite. One important application of such material composites is in rotary anodes for X-ray tubes.
A good high temperature-proof bond between the graphite and the high-melting metal is an important factor relating to the usability of material composite structures. When heat has to be rapidly dissipated from the high-melting metal, as for example in rotary anodes, the bond between the high-melting metal and the graphite should have fairly good thermal conductivity in order to exploit the high heat storage capacity of the graphite. In addition, when using carbide-forming high-melting metals, as little carbon as possible should diffuse from the graphite into the high-melting metal at the high application temperatures in order to prevent the high-melting metal from forming carbide. Formation of carbide causes brittleness and thus deterioration of the resistance to thermal shock of the composite body, as well as deterioration of the good thermal conductivity of the carbide-forming high-melting metal itself.
Various designs of composite bodies made of graphite and carbide-forming, high-melting metals are known in the art. One embodiment is that of a solid basic body of graphite, which is joined with a thin coating of carbide-forming, high-melting metal, the thickness of the coating being in the order of magnitude of up to 1000 .mu.m. If the coating of the carbide-forming, high-melting metal were applied to the graphite directly, then the high temperatures generally prevailing during operation of the composite body would cause the high-melting metal to convert into a carbide through diffusion of carbon from the graphite. This means that a single or multi-layer intermediate layer has to be arranged between the graphite and the carbide-forming, high-melting metal. The intermediate layer serves as a barrier for the diffusion of carbon. Such barriers are known. As a rule, the intermediate and the top or cover layers of the carbide-forming, high-melting metal are applied to the graphite by special coating processes, e.g. the CVD or the PVD-process.
For example, it has already been proposed to arrange a single-layer intermediate layer of tantalum between a basic body made of graphite and a tungsten layer. At the high operating temperatures of the composite body, the tantalum layer is transformed into tantalum carbide through carbon diffusion from the graphite. The tantalum carbide then forms a diffusion barrier preventing any further diffusion of carbon into the tungsten layer. The intermediate layer has to be of adequate thickness in order to safely avoid the carburization of the tungsten layer even under extreme operating conditions in terms of temperature and time. However, due to the brittleness of the tantalum carbide and the differing coefficients of thermal expansion of the individual materials, whose effect becomes stronger with increasing layer thicknesses, the layered composite often becomes detached from the basic graphite body when the composite body is stressed by thermal shock.
DE-OS 22 63 820 describes the arrangement of a two-layer intermediate layer in a rotary anode with a basic body of graphite and a burning track layer consisting of carbide-forming a high-melting metal, for example tungsten. The layer of the intermediate layer that is directly arranged on the graphite consists of a metal not forming carbide such as iridium, osmium, or ruthenium. The second layer of the intermediate layer consists of a carbide-forming, high-melting metal such as hafnium, niobium, tantalum or zirconium.
The second layer, which at the operating temperature of the rotary anode is transformed into a brittle carbide through diffusion of carbon from the graphite, is joined with the graphite body via the metallic and thus plastic layer. This plasticity of the metallic layer is expected to compensate for variations in the expansion of the individual materials under thermal stress, and therefore is expected to obtain good adhesion of the burning track layer to the graphite body. In practice, however, a gradual but constantly increasing carburization of the burning track occurs over time in spite of the presence of the intermediate layer. This in turn leads to early failure of the rotary anode due to detachment of large-size parts of the burning track from the graphite, or as the result of particles which chip off the burning track.
EP-PS 0 023 065 describes a rotary anode for X-ray tubes. The anode consists of a solid basic graphite body and a thin burning track made of a high-melting metal, the latter being applied to the basic graphite body via a three-layered intermediate layer. The individual layers of this intermediate layer abutting the basic body and the burning track consist of pure rhenium. An additional layer consisting of an alloy of rhenium with at least one carbide-forming metal such as tungsten, tantalum or hafnium is arranged between these layers. One drawback of this embodiment of a rotary anode is that at the temperatures used in the application of the coating, or prevailing when the anode is operating, a compound is formed in the layer consisting of the alloy of rhenium with the carbide-forming metal which is brittle. In the case of tungsten-rhenium, a so-called sigma-phase develops, which has a highly unfavorable thermal conductivity that is much lower when compared with pure tungsten. As the sigma-phase extends across the total thickness of the coating after a longer time of use, these rotary anodes also break down early due to the mechanical or thermal failure of the burning track.
Another rotary anode for X-ray tubes is also described in JP-A 59-114739, in which a three-layer intermediate layer consisting of a first layer of rhenium, a second layer of a carbide, e.g. molybdenum carbide, and a third layer of rhenium again, is arranged between the basic graphite body and the target consisting of high-melting metal. The rhenium of the first layer and the carbide of the second layer are deposited on the basic graphite body from the gas phase. The rhenium of the third layer is formed by a foil or sheet, which for the manufacture of the composite body, is inserted between the coated basic body and the target. This structural arrangement of the intermediate layer also cannot completely prevent the carburization of the burning track.
Another known embodiment of a composite body consists of a solid basic body made of graphite and a solid part consisting of a carbide-forming, high-melting metal, the latter being joined with the basic body by soldering or diffusion welding via one or several intermediate layers. In most cases, composite bodies consisting of a solid basic body of graphite and a solid part made of a carbide-forming, high-melting metal are manufactured by arranging the solder or connecting material between the two parts without any additional intermediate layer, and then by joining the parts by melting the solder or by combining the parts by hot pressing. Such embodiments are described, for example in DE-B-12 25 023, EP-B-0 037 956, DE-B-21 15 896, and DE-C-27 48 566. One drawback with these composite bodies is that the solder material may to some extent carburize, the result being the negative phenomena heretofore described. Additionally, with these embodiments relatively thick layers of the material which does not form a carbide such as rhenium often have to be used in order to safely exclude the formation of carbide in the high-melting metal part. However, this is not justifiable in terms of cost in many cases. Furthermore, during the soldering or hot pressing, degassing of the graphite may occur which will promote the adverse formation of blowholes in the intermediate layer.
U.S. Pat. No. 4,777,643 describes a rotary anode consisting of a basic graphite body and a part made of a high-melting metal which supports the burning track, the two parts being joined via a diffusion bond. In order to avoid the formation of carbide in the high-melting part to the greatest possible degree, the parts are joined through the use of an intermediate layer consisting of platinum or a platinum alloy and an intermediate layer consisting of tantalum or niobium, or alloys thereof. This design, however, also does not adequately meet all operating requirements for rotary anodes.
The deposition of pure rhenium or rhenium-containing layers from the gas phase has been known in connection with rotary anodes for a long period of time and has been described, for example in DE-B-11 06 429, or AT-B-278 184.